Drones are provided with multiple rotors driven by respective motors that are independently controllable in order to control the drone in attitude and in speed.
A typical example of such a drone is the AR. Drone from Parrot SA, Paris, France, which is a quadricopter fitted with a set of sensors (altimeter, three-axis gyros, accelerometers). The drone also has a front camera picking up an image of the scene towards which the drone is heading, and a downward-looking camera picking up an image of the terrain being overflown.
The drone is controlled by the user by means of a remote control device that is connected to the drone via a radio link.
WO 2010/061099 A2 (Parrot SA) in particular describes such a drone and how it can be controlled by means of a telephone or a multimedia player having a touch screen and an accelerometer incorporated therein.
If the motors are controlled in such a manner as to cause the drone to tilt or “dive” nose-down (tilt with a pitching angle), then it will move forwards at a speed that increases with increasing tilt angle; conversely, if it takes up a “nose-up” position in the opposite direction, its speed is slowed down progressively and then reverses, going off rearwards. Similarly, titling about a roll axis (with the drone leaning to right or to left) causes the drone to move horizontally in linear manner to the left or to the right.
In general, the term “tilt” is used to mean the drone being tilted relative to a horizontal plane of a fixed terrestrial frame of reference, it being understood that the longitudinal and transverse components of its horizontal speed and its tilt about the pitching and roll axes respectively are intimately associated
The drone is also provided with an automatic stabilization system that serves in particular to enable the drone to reach an equilibrium point automatically and, once the equilibrium point has been reached, it provides the corrections needed to maintain a point that is stationary, i.e. by correcting small variations of movement in translation due to external effects such as movements of the air. During this stage, sensor drift is estimated by trimming.
The inertial sensors (accelerometers and gyros) serve to measure fairly accurately the angular speeds and the attitude angles of the drone (i.e. the Euler angles describing the tilt of the drone relative to an absolute terrestrial frame of reference). The signals they deliver can thus be used for dynamically servo-controlling the thrust direction of the drone to the direction opposite to that of the disturbance or to the direction opposite to the piloting commands sent to the drone by the user.
The altimeter is an ultrasound telemeter located under the drone and it delivers an altitude measurement that enables the thrust force to be servo-controlled in order to stabilize the drone in height.
Linear speed in a horizontal plane (speed of the movement in translation of the drone as represented by two orthogonal components extending longitudinally and transversely in a horizontal plane of a terrestrial frame of reference) is evaluated by analyzing the image delivered by the downwardly-looking camera of the drone in combination with accelerometer data, using software that estimates movement from one image to the next in the scene picked up by the camera, with this estimated movement being subjected to a scale factor that is a function of the measured altitude. Various algorithms make it possible to determine in real time the horizontal speed with good accuracy, both for values that are close to the maximum speed of the drone, which is of the order of 8 meters per second (m/s), and for values that are very small, around the equilibrium point in a hovering flight configuration (in this configuration, the inexpensive accelerometers that are used generally suffer from too much noise to give a satisfactory estimate of the speed of the drone after double integration of the signal, so measuring speed by means of the camera makes it possible to compensate for the errors of these sensors).
More particularly, the invention relates to the transition:                from a state in which the drone is flying at a high speed (and thus with a non-zero tilt angle), referred to below as a “moving state”, and defined by the piloting commands sent to the drone by the user;        to a state in which the drone is not moving, referred to below as a “hovering” state, in which the horizontal speed of the drone is zero and its tilt angle is likewise zero. In this state, the loop for automatically stabilizing the drone in hovering flight is activated in order to maintain this hovering state at a speed and an angle of inclination that are both zero, as explained above.        
Such a transition occurs in particular when the user triggers a changeover from a controlled mode of piloting in which the drone maneuvers in response to commands applied by the user via the control appliance, to an autopilot mode in which the drone maneuvers solely on the basis of data picked up by its sensors, without intervention on the part of the user.
As explained in above-mentioned WO 2010/061099 A2, this transition occurs in particular when the user “lets go” the controls, i.e. takes the fingers off the touch screen of the appliance: under such circumstances, and for safety reasons, the drone is brought automatically into a hovering state of flight.
In order to perform this transition, the autopilot system applies a setpoint to the loop for controlling the motors of the drone in such a manner as to reach the zero target values for speed and for angle of tilt.
Nevertheless, if the zero target values for speed and for angle of tilt are applied directly as setpoints to the control loop, it is often observed that the drone takes quite a long time to reach the final hovering state after following a path that is long, and often after its speed has changed sign one or more times, i.e. the drone overshoots the fixed point, reverses, oscillates, etc., with this involving a stopping time (i.e. the time taken to reach the final hovering state and as measured from the beginning of the transition) that is not optimum when compared with a situation in which a skilled user directly controls the transition from the moving state to the hovering state.